Aptamers are single-stranded DNA or RNA molecules which can bind a variety of chemical and biological molecules with high affinity and selectivity. [1-4] They are isolated from a large random pool of DNA or RNA molecules using a combinatorial biology technique called systematic evolution of ligands by exponential enrichment (SELEX) [1,2]. They are often comparable to antibodies in their selective and sensitive binding to a broad range of molecules [5-8]. The major advantage of these molecules lies in the relative ease with which they can be selected for any target analyte and their stability against biodegradation and denaturation. Due to these properties aptamers are good candidates for making chemical and biological sensors in many fields such as medical diagnostics and environmental monitoring. Therefore, these aptamers have been converted into fluorescent [9-22], colorimetric [23-29] and electrochemical sensor [30-33].
For example, U.S. Publ. Pat. No. 20040175693 makes use of the discovery that the cleavage of a nucleic acid substrate by an aptazyme upon binding of an effector can be detected calorimetrically. In the presence of the effector, the substrate is cleaved and aggregated particles are dispersed, resulting in a color change. This system combines the benefit of elements that can recognize any molecule of choice with high sensitivity and ease-of-use provided by calorimetric detection.
While the above aptamer sensors have been widely explored in vitro (See, for example, U.S. Publ. Pat. No. 20030215810), their applications in vivo, particularly in humans, remain a significant challenge because of the difficulty light has in penetrating through skin and signal interference from cellular components.
Magnetic resonance imaging (MRI) is a powerful method for non-invasive three-dimensional imaging of cells and human bodies that is at the base of imaging techniques such as differential tensor imaging (DTI). One active area of research in this rapidly advancing field is development of novel MRI contrast agents, particularly smart agents that are responsive to small or biomolecular markers in cells or human bodies before cellular components or tissues display any MRI differences.